Are you using a clutch rear mech?
No, just a standard Tiagra rear mech. I know I could get clutched, but intention was a cheap set up. It not a regular thing, just annoying when it happens.
I had the same step up, end up shortened the chain which stopped it coming off.
Also; freehub body, some freehub body are a bit stiff which at speed, can slacken the chain result in it coming off.
@efb started
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Are you using a clutch rear mech?